Coated steel article

ABSTRACT

A steel substrate having an intermediate adherent coating of gold or silver or copper or brass, and an outer adherent coating of zinc, in which the iron content in the coatings, occasioned by alloying between the iron and the coatings thereon, is less than about 100 milligrams per square foot of surface.

United States Patent 11 1 Shepard et al. 1 1 Oct. 2, 1973 COATED STEELARTICLE 2,490,700 12/1949 Nachtman 29/1963 2,547,947 4 1951 K1 1 29196.3 [75] Inventors: George A. Shepard, Parma; Carl F. 3,164,448 111965p z 29;196.3 Br k n fi g ts th f 3,249,409 5/1966 McLeod.. 29/1963 Ohio3,555,169 1/1968 Miller 29/1963 [73] Assignee: RepublicSteelCorporation,

Cl la (1, ho eve n O 1 Primary ExaminerHyland Hum [22] led: 1970Attorney-Cooper ct al. [2]] App]. No.2 86,623

Related US. Application Data [60] Division of Ser. No. 680,607, 061. 20,1967, 1 1 ABSTRACT abandoned, which is a continuation-in-part of Ser.No. 423,249,1an. 4, 1965, abandoned. A steel substrate having anintermediate adherent coating of gold or silver or copper or brass, andan outer ad- [52] US. Cl 29/l96.3, 29/199, 29/194 herent coating ofzinc, in which the iron content in the [51] Int. Cl. 823p 3/00, B32b15/00 coatings, occasioned by alloying Ibetween the iron and [58] Fieldof Search 29/1963 the coatings thereon, is less than about 100milligrams per square foot of surface. [56] References Cited UNITEDSTATES PATENTS Kenmore 29/1963 1 Claim, N0 Drawings COATED STEEL ARTICLECROSS-REFERENCE TO RELATED APPLICATION This application is a division ofour copending application Ser. No. 680,607, filed Oct. 20, 1967 nowabandoned, for COATING OF ZINC ON STEEL, which in turn is acontinuation-in-part of our copending application Ser. No. 423,249 filedJan. 4, 1965, now abandoned for COATING OF ZINC ON STEEL.

This invention relates to vacuum metalizing of steel and is particularlydirected to the provision of an adherent coating of zinc on steelsurfaces by deposit from vapor of metallic zinc in a vacuum.Theinvention is primarily concerned with improved methods of providingsuch zinc coatings of steel by vapor deposition, and likewise to theresulting articles, such as steel sheet carrying an adherent, smoothzinc coating of the character stated.

Zinc-coated steel sheet, meaning strip or other sheet of steel, whethercold rolled or hot rolled, has distinct advantages for many fields ofuse, particularly in avoiding or reducing corrosion or likedeterioration of the steel surface. While galvanized steel has beenwidely used for a long time, i.e., steel having a zinc coating appliedby conventional hot-dip galvanizing methods, :the nature of the surfaceof such coatings has prevented their employment in many circumstances,especially automotive work, e.g., as for automobile bodies, especiallybecause of the relatively rough nature of the galvanized layer. Therequirements of automobile paints or other finishes to be applied oversuch steel sheet, particularly the need for a highly smoothproduct, haveprevented successful use of galvanized sheet in this and similar fields,in most cases. While electroplated zinc can provide a satisfactorilysmooth finish, zinc electroplating operations are relatively costly andsteel so coated has been unattractive to manufacturers in automotive andother industries for this reason.

Vacuum metalizing, involvingthe deposit of a metal from its vapor, isknown to produce surfaces of good smoothness and can usually beperformed more .economically than procedures of electrolytic deposition,for coatings of significantly usefulthickness. Moreover, such thicknesscan be readily controlled to any desired value in vapor depositionmethods, as distinguished from hot-dip operations where itis difficultor impossible to apply anything but a relatively heavy layer, which maybe needlessly excessive, and thus unduly costly for many purposes.Although satisfactory coating on steel has been achieved by vapordeposition of a number of metals, of which an outstanding example isaluminum, it has unfortunately been found that zinc layers applied inthis manner tend to have poor adherence. Even with unusually thoroughcleaning operation prior to the vacuum metalizing treatment, zinc layersthus applied to steel have been found to be subject to removal, i.e., byflaking or peeling off, on bending or otherwise flexing the surface.This difficulty is critical, especially for automotive and like uses, inthat steel so employed must be bent or shaped to any of a wide varietyof configurations, often involving a relatively deep draw in the formingprocess, to the extent of including some actual deformation or extensionof the metal.

The present invention is therefore designed to pro-, vide for theapplication of zinc coatings, over steel articles, by vapor depositionin vacuum, and to achieve products consisting of articles so coated,wherein the zinc layer, of essentially any desired thickness, is firmlyand permanently adhered, against removal even upon severe bonding,distortion or similar manipulation of the sheet or equivalent product.To these ends, the method of the invention involves treating the steelsurface, after cleaning appropriate for vacuum metalizing, by applying athin adherent coating of copper or other metal selected from a specificclass of metals in the upper range of the electromotive force series,namely gold, silver and copper, brass being also deemed a member of thenamed group, in that the latter is an alloy consisting essentially ofcopper and zinc wherein copper predominates. A further, principal stepof the method thereafter involves applying .a coating of zinc over thethin copper or like film, by deposition from zinc vapor in vacuum, toachieve the desired thickness. By virtue, apparently, of some mutualpenetration between the coatings, or at least of some close associationor engagement of the molecules of zinc upon and around the molecules ofthe selected underlying metal, full and effective adherence is achieved,the underlying metal being such as to be inherently characterized byeffective adherence, whether by inter-alloyingor other wise, to theactual steel surface.

While the first-applied film orcoating, e.g., ofcopper or brass, canitself be achieved by vapor deposition,.a further andparticularlyadvantageous feature of the invention involves formation of this firstcoating by pionic deposition from an aqueous medium, whetherby simpledisplacement or by electrolytic action. Thus for example the cleanedsteel surface may .be immersed in a solution of a copper salt, such ascopper sulfate, whereupon a thin,effective film of copper isdepositedpursuant to the well known reactioniof displacementofthe copper ions byiron. A lternatively,,and with special advantage inmany cases, thecopper may be deposited by a brief electroplating operation, i.e.,directly on the steel surface. Copper coatings applied by either .ofthese methods afford highly satisfactory adherence of thevapor-deposited zinc layer subsequently applied thereon. They may bevery readily controlled as to thickness, i.e., in the sense that only avery thin copper filmis usually necessary. Since only a verybriefchemical-type treatment is required, the cost is relatively smallwhether by immersion or by electroplating, and indeed this operation maysimply be incorporated in the so-called cleaning line, e.g.,.as one ofthe finalsteps after the steel strip or other sheet hastraversed thevarious baths and rinses required toremove oxid,e,.grease, dirt and thelike.

Following theapplication ofthe copper, brass or similar film (itbeingunderstood that brass may likewise ,be plated, by conventionalmethodslall that is necessary is that the surface be rinsed, as in plainwater, and dried, before the vacuum metalizing step. Of course,

the copper orsimilarly coated sheet should be maintainedin a dust freecondition, and under such circumstances, orfor no more than a brieftime, as to avoid appreciable oxidation. The vacuum coating with zinc isthenperformed in accordance with conventionalmetalizing procedure,preferably by directing a stream of zinc vapor into the coating chamberand upon the surface of the treated steel, for example as where suchsteel strip, wire or other article is continuously advanced. The zincdeposits in solidified, continuous form as ahighly smoothcoating andmaybe applied to any desired thickness, at least up to the point where itmight tend to exist or remain in molten form. Thus for instance, zinccoatings of a thickness of about one mil are quite satisfactory,providing corrosion resistance equivalent to conventional hot-dipgalvanized coatings. The resulting completed steel articles being sheet,strip, plate, wire, bar, rod or other shape, are characterized byexcellent adherence of the zinc layer, as well as by complete smoothnessof the outer surface, free of the so-called spangled or crystalline orotherwise relatively rough nature which is an attribute of galvanizedproducts. The reason for the unusual result of the process and theeffective adherence of the coating in the produced article, is not fullyunderstood, especially in view of the fact that a'number of othermetals, such as aluminum, can be found to adhere very well, on directapplication to steel. It is now believed that the problem of adherenceof vapor-deposited zinc may occur because the molecules of zinc vaportravel at relatively low speeds, and because the zinc molecules, forthat or other reasons, may tend to condense less readily. It is nowunderstood that aluminum molecules travel at about twice the speed ofzinc; hence it is conceivable that the correspondingly low momentum ofthe zinc vapor is a factor in its ordinarily poor adherence. in the samesense, there may be some tendency of the depositing zinc molecules orparticles to bounce away from the surface, in contrast to the situationin the condensation of aluminum vapor. In any event, it is plain thatwith the first applied film of copper (or other metal of the specifiedclass) over the steel surface, the zinc vapor condenses in a fullyadherent state, apparently with some penetration of the zinc moleculesinto the copper as explained above. Indeed when very thin layers of zincare applied over a copper film, the resulting surface has a yellow orbrass-like color, evidencing such inter-penetration, and lendingcredence to the theory that the copper film permits immediate embedding,so to speak, of the slow-moving zinc molecules in a manner unattainableon a bare steel surface. It is believed that some alloying of the vapordeposited zinc with the copper or the base metal is necessary for goodzinc adherence. The iron content in the alloy layer should not exceedabout 100 mg. per square foot of surface, however.

While it was initially thought that the plural coating structure mightconstitute a copper-zinc couple and would lead to corrosion of the zincin an accelerated way, tests have shown that no such result appears tohappen, i.e., under conditions of exposure that might give rise to it.The desireable avoidance of such results is also not fully explained,although it may be that the copper-zinc alloy serves as a buffer.Dealing further with the mechanical situation of the coating, a furtherpossible comment is that the under-layer is constituted of a metal whichforms alloys at low temperature, as contrasted to steel and iron, andwhereas the condensing zinc vapor may not sufficiently heat up a baresteel surface to form an interfacial alloy, such alloying takes placemuch more readily with the copper, while the copper, in turn, isinherently able to engage the steel surface in a very close and thussuitably adherent manner.

As indicated, the coating of copper, brass, gold or silver, which isapplied directly to the steel surface, is relatively thin, as forexample from 0.25 X inch, and preferably from 0.5 X 10 inch to anyconvenient and economically practical thickness, e.g., 100 X 10" inch.

Indeed, in some cases the coating can be even thinner, presumably nomore than a few molecules in thickness (although a single molecularlayer presently seems to be inadequate), especially in the case of goldand apparently also under some circumstances with silver and copper.While films thicker than the upper limit named above, being one tenthousandth of an inch, are presumably not deleterious, reasons ofeconomy dictate the use of as thin a layer as possible. A film 5.0 X 10inch in thickness, or ordinarily much less, is believed to be fullyadequate; a film thickness of about 4 X 10 inch is preferred.

As stated, the copper, brass or other layer can be applied in variousways, but ionic deposition from solution seems unusually convenient andadvantageous. Simple immersion plating is quite effective, although someadjustment of the solution used, as in pH, may be necessary to achieveoptimum results, while avoiding waste of copper. That is to say,immersion plating depends on the activity of the steel surface, whichmay vary with different steels and with different surfacecharacteristics. In contrast, electroplating ordinarily involves no suchdifferences, and can be controlled in conventional ways to deposit acoating of any desired thickness. In the case of gold, silver and brass,electroplating is similarly effective, for applying corresponding filmsof selected character, by conventional techniques. Metals of less noblecharacter than those named appear to be ineffective to constitute theunderlying coating, i.e., in that no increase of adhesion of zinclayers, deposited from vapor, has been found with metals such ascadmium, iron or lead, nor any significantly satisfactory effect withnickel or tin. Aluminum similarly appeared to contribute no improvementin adherence of the zinc nor was it found of any advantage to treat thesteel with a flash electroplating of zinc.

A further advantage of the process, in contrast to hot-dip galvanizing,is that the zinc coating can, if desired, be applied on only one surfaceof the metal sheet or like article, thus greatly economizing theoperation where only a single face requires protection.

The vapor depositing step for applying the zinc layer, e.g., over thecopper or similar film, can be carried out in any satisfactory manner,as in conventional vacuum metalizing equipment. Preferably, the zincvapor is generated in a separate chamber and directed as a stream towardthe passing steel strip or the like in the main evacuated region, e.g.,under pressures from 20 to 30 microns down to one micron or less;pressures of 20 microns or less are preferred. The rate of vapor flowand the time of exposure, i.e., speed of strip travel, are appropriatelycorrelated to the desired thickness of zinc coating as will be readilyunderstood in the vacuum metalizing art. It should be noted that higherzinc condensation rates are preferred over lower zinc condensation ratesduring zinc vaporization. The coppercoated steel surface should bemaintained at a temperature within the range 300 to 600 F. duringvaporization of the zinc coating in order to ensure good adherence ofthe coating. The zinc vapor may raise the substrate to the desiredtemperature; in the case of thin zinc coatings, however, it may benecessary to preheat the substrate to ensure the desired temperature.Preheating the copper-coated steel surface may expand the substratetemperature range, e.g., so that the range may be from 250 F. to 660 F.Post heating the zinc coated article may enhance adherence of thecoating.

As has been indicated, the zinc layer may have any desired thickness,although the invention is particularly suitable for such coatings in therange of 25 X to 2000 X 10 inch and most preferably 100 X 10' to 1000 X10 inch (0.1 to 1 mil). Present understanding is that a zinc coating ofabout one mil thickness affords adequate protective function forordinary automotive uses.

The invention is applicable to a wide variety of steels, includingparticularly those most commonly employed in automobile body work andthe like. Examples of these and other steels are as follows: low carbonaluminum killed steel; low carbon (mild) steel; and low-carbon siliconekilled steel. The process is also appropriate for various specialferrous alloys.

If desired, one of the zinc coatings may be omitted, as by simplydirecting vapor to one face of the sheet during its travel through thevacuum chamber. The copper film may also be omitted from such face,although immersion techniques may make it simpler to coat both sides.

By way of specific example, steel sheet material has been treated inaccordance with the described process, using various coatings of thenamed metals directly on the steel surface, followed by vacuumdeposition of zinc from a stream of zinc vapor. In general, the steelused was low carbon, cold rolled steel strip having a gauge or thicknessof 0.030 inch, but tests have indicated thatthe process is equallyapplicable to steels generally, as explained above, with essentially nomodification in detail, except for instance in the situation where acopper coating is applied by simple immersion plating. In the latterinstance, the time of immersion and the acidity of the plating solutionmay require adjustment for best efficiency, as will be readilyunderstood or as can be easily determined in any given case.

Preliminary to the treatment, the steel surface or surfaces should beappropriately cleaned, for example as by any series of operationsgenerally suitable as preparation for vacuum metalizing. One effectivecleaning sequence is described in U.S. Pat. No. 2,959,494, issued Nov.8, 1960 (G. A. Shepard), such operations being abundantly suitable toclean the steel surface appropriately for the coatings of the presentinvention, in lieu of the aluminum coating primarily contemplated by thepatent. Indeed the sequence of soak-cleaning, electro-cleaning(cathodic) and acid pickling described in the patent may usually besufficient for present purposes, without the final alkaline soak stepthere described. That is to say, in such case, the pickled strip orsheet is then rinsed and immediately passed through the immersionplating or electroplating tank, for deposition of the first metalcoating. After such treatment, the surface or surfaces are rinsed withwater and dried, and can be immediately subjected to the step of vapordeposition of zinc.

Continuing the description of examples of the process, immersionplatings of copper were achieved by exposing the surface or surfaces toa water solution of suitable copper salt, e.g. copper sulfate, in aconcentration of 7.5 grams/liter, adjusted to a pH of about With theselected steel, an adequate copper coating of about 0.5 X 10' inch wasachieved on immersion for 2 seconds, and even better results wereobtained in some cases with somewhat thicker coatings, up to 1.2 x 10inch, by leaving the steel in solution for periods up to 4 seconds.Immersions for times substantially shorter, yielding copper films havinga thickness believed to be of the order of several molecules, were alsoemployed, with significantly useful results as to adherence of thelater-applied zinc coating, from zinc vapor.

Electroplated copper coatings were likewise achieved by subjecting thesteel sheet to a conventional plating operation, utilizing pure copperanodes and an aqueous solution having the following composition: coppercyanide 3.0 oz./gal., sodium cyanide 4.5 oz./- gaL, sodium carbonate 2.0ozJgal. (room temperature). Plating was carried out with a DC currentdensity of about 15 amperes per square foot of steel surface, andsatisfactory copper coatings were achieved, having thicknesses rangingfrom 0.5 to 50 X 10* inch, with plating times ranging from 2 seconds to2 minutes. Again, somewhat thinner coatings down to several moleculeswere achieved with a still shorter duration of electroplating.

Brass coatings, having a composition of approximately copper and 20%zinc, were also applied to surfaces of the steel strip by conventionalelectroplating procedure, utilizing anodes of brass having the statedcomposition. A solution used had the following composition: coppercyanide 4 02., zinc cyanide 1.25 02., sodium cyanide 7.5 02., sodiumcarbonate 4 02., water one gallon. With plating current of 5 amperes persquare foot of the steel surface, through the aqueous solution, suitablebrass films were adherently applied, having thicknesses from 0.5 to X 10inch, with plating times of 5 seconds to 10 minutes.

Silver and gold films were also respectively applied to furtherspecimens of steel, utilizing conventional plating solutions. In thecase of gold, the solution contained 4 o2./gal. of potassium goldcyanide, and for silver plating, the composition of the solution was 4g./liter of silver cyanide. With current densities of about 20 amperesper square foot of the steel surface, highly effective coatings of goldand silver were respectively achieved, having thicknesses from 0.25 to100 X 10 inch. In these cases, some extremely thin films were alsoproduced and found to afford satisfactory results, especially in thecase of gold where a film believed to be no more than a few moleculesthick was achieved with a flash plating and subsequently found to affordadherence of the vapor-deposited zinc.

Sections of steel strip plated in all of the various ways describedabove were rinsed, dried and subjected to vapor deposition of zinc undervacuum conditions, e.g. usually a pressure of not more than 20 to 30microns. It was found that the conditions of zinc coating could be thesame for the various precoated specimens, the chief adjustment in thisstep being with respect to the intensity and duration of the vaportreatment, for the attainment of different thicknesses of zinc. Thusquite satisfactory results were obtained by exposing the precoated steelsurface to a flow of zinc vapor, evaporated at about 1350 F. and havinga delivery rate of about 0.07 to 0.12 pounds of zinc per minute. Theactual deposit of zinc on the surface of the sheet or strip was at therate of 2.5 to 26.0 pounds per square foot per min ute, for theachievement of zinc coatings having thicknesses ranging from 0.1 mil to1.0 mil over exposure times of 1.5 to 8 seconds.

Within the ranges specified by example above, all of the steel stripfirst coated respectively with copper (by immersion or electroplating),brass, silver and gold, and thereafter subjected to vapor deposition ofzinc to thicknesses of the range indicated, were found to affordexcellently adherent zinc coatings. Adherence was tested by flexing thestrip back and forth several times, or indeed even many times, therebeing no evidence of removal of any part of the zinc layer. In contrast,steel strip directly subjected to the same zinc vapor deposition,yielded zinc coatings which quite often commenced to flake or peel away,at the zones of flexure, even after no more than a few bendingoperations. Another suitable test for adherence of vacuum-depositedcoatings, such as the zinc coatings of the present invention, is setforth in the above-mentioned US. Pat. No. 2,959,494 (column 5, lines 13and following), such test involving applying a lacquer over the coating,then pushing a ball into the steel sheet, while the latter is drawn overthe ball to the point of rupture. If a pressure-sensitive adhesive tapeapplied over the drawn area, and thereafter removed, shows that it hascarried away pieces of the zinc coating, adherence is generallyinadequate. This test, which has shown good adherence with the variousexamples of the present invention, is significant in demonstrating theability of the coated product to withstand forming operations in use.

In all cases of the exemplified procedure, the resulting new product ofsteel carrying a vapor-deposited coating of zinc had effectively usefuladherence of such zinc coating, and in its structure, consistedessentially of the underlying steel, carrying the defined film ofselected first-coating metal, and thereover the protective coating ofzinc applied by vapor deposition.

In tests conducted employing coatings of copper over steel plate,followed by a coating of zinc, it was found to be advantageous to havesome alloying of zinc with the copper or underlying steel. Forrelatively thick copper coatings, the zinc will alloy with the copperonly and will not penetrate the copper sufficiently to alloy with theunderlying steel. In the case of thinner copper coatings, however, thezinc will alloy with the copper to form brass and will also alloydirectly with the steel; also, the brass so formed will alloy with thesteel. It has been found that although alloying is desirable, generallythe iron content in the alloy should not exceed 100 milligrams persquare foot of surface. Excessive alloying (greater than 100 mg.lft.")has generally resulted in poor adherence of the zinc coating.

To provide proper alloying of the zinc with the copper or steel, it hasbeen found that the substrate should reach a temperature during the zincvaporization step that is generally within the range 300 to 600 F.Examples of steel plate coated with copper followed by vapor depositionof zinc in vacuum have produced good adherent coatings when thesubstrate reached a maximum temperature within this range. If arelatively thin coating of zinc is being applied, the zinc vapor may notraise the temperature of the substrate sufficiently so that thetemperature comes within the desired temperature range of 300 to 600 F.In such a case it may be necessary to heat the substrate independently,e.g., by electrical resistance heating in order to ensure that thesubstrate reaches a temperature within the desired range.

Samples of steel plate coated with copper of a thickness ofapproximately 4 X inch, followed by glow discharge cleaning for 30seconds and then plating by zinc vapor in vacuum, produced zinc coatingsof excellent adherence when the maximum temperature of the substrateduring evaporation was within the range 300 to 600 F., starting with asubstrate at ambient or room F. showing an iron content in the alloylayer of I00 milligrams per square foot or less all produced zinccoatings of excellent or good to excellent adherence. It is believedthat the glow discharge cleaning is not necessary for adherent coatings.

It has also been found that pre-heating the substrate prior to thedeposition of zinc on the copper coating extends the temperature rangejust referred to by 50 or 60 F. on either end of the range so that therange for good plating results is from 250 to 660 F. It is believed thatpre-heating the substrate extends the range at the lower end,particularly, i.e., to 250 F., because alloying of the zinc vapor withthe copper or underlying steel may take place at temperatures lower than300 F. due to the relatively high energy of the zinc vapor. If thesubstrate is not pre-heated, alloying will not take place between thezinc and the substrate when the vapor initially deposits until thesubstrate is increased in temperature by the action of the zinc vapordepositing on the substrate. In this regard, the deposited zinc is of alower energy than the vapor prior to deposit, and hence must besubjected to a higher substrate temperature in order to produce thedesired alloying between the zinc and substrate. It is believed, then,that by preheating the substrate, proper alloying during the zinccoating step may take place at a lower temperature with the higherenergy zinc vapor.

Samples of steel plate coated with copper were produced in which thesteel base was first cleaned and then approximately 4 X 10' or 12 X 10inch of copper was deposited upon the cleaned steel base. Differentsamples of both copper thicknesses were subjected to preheating withinthe temperature range 300 to 750 F. (in increments of F.). Followingthis, and while the substrate was still hot, the zinc was vapordeposited upon the pre-heated copper coated steel to produce relativelythick zinc coatings (over 0.5 X 10 inch in thickness). For each sample,the substrate temperature during vaporization of zinc varied over arange that encompassed the pre-heat temperature. Typically, thesubstrate temperature during zinc vaporization for each sample variedfrom below the pre-heat temperature to 200 above the pre-heattemperature. For example, in the case of a sample coated with 4 X 10inch of copper and pre-heated to approximately 500 F., vaporization ofzinc took place within a range of 275 to 600 F. Zinc coatings ofexcellent or good adherence were generally produced for samplespreheated to between l50 and 600 F.

Samples similar to those just described were preheated to temperatureswithin the range to 660 F. (in increments of 25 to 100 F. for thedifferent samples). Each copper plated sample was cleaned by glowdischarge for 30 seconds (glow discharge cleaning is not believednecessary for adherent coatings) and was zinc vapor plated within arange having as a lower limit the pre-heat temperature and an upperlimit approximately 50 higher than the pre-heat temperature. The samplespre-heated to between 225 and 660 F. all provided zinc coatings havingexcellent adherence (thin coatings, less than 0.5 X 10" inch thick).

It has also been found that higher condensation rates in theorder of 608X inches/min. compared with lower condensation rates in the order of 64X 10 inches/min. provide better coatings of zinc on copper over steel. Anumber of steel bases were cleaned and plated with copper and thencoated with zinc by vapor deposition in vacuum in which the rate ofvaporization varied. None of the samples was pre-heated; each sample wasbrought to a temperature within the range approximately 300 to 600 F.(in increments of roughly 50 F. for the different samples). All thesamples coated with zinc at a relatively high condensation rate (600 X10 inches per minute) provided zinc coatings having excellent adherence.Samples coated with zinc at a relatively medium condensation rate (250 X10 inches per minute) provided coatings of excellent adherence exceptthose deposited at temperatures of 300 to 350 F. Samples coated withzinc vapor at a relatively low condensation rate (60 X 10 inches perminute) provided coatings of excellent adherence only within the range400 to 500 F. As a result, it is be-- lieved desirable to vaporize at arelatively high condensation rate, in which case vapor condensation cantake place over a substrate temperature range from 300 to 600 F. or 250to 660 F., as noted above.

The time during which vaporization of the zinc onto the copper takesplace is not critical except in its effect upon the raising of thetemperature of the steel substrate. As the vapor is deposited on thesubstrate, the temperature of the substrate is raised. The time ofvaporization thus should not be so long as to raise the temperature ofthe substrate above the 600 or 660 F. upper limiting temperaturesreferred to above.

It has been found that strongly adherent coatings are produced when thevacuum level is maintained at 20 microns of mercury or less. Samples ofsteel base were cleaned and copper plated and then coated with zincvapor under vacuum; for each sample the substrate temperature variedfrom room temperature to approximately 450 F. during vaporization. Forthose samples of which the vacuum level was maintained under 20 microns, zinc coatings were produced having excellent adherence.

It has been found that post heating of the zinc and copper coated steelplate improves the adherence of the zinc coating. Samples of steel platewere prepared with approximately 4 X 10 inch of copper. The zinc wasvapor deposited in vacuum at a substrate temperature of approximately250 F. In all cases the initial adherence of the zinc coating was poor.The samples were then post heated in air for 5 minutes to temperaturesin the range 300 to 750 F. (in increments of generally 50 or for thedifferent samples). The poor adherence was improved by post heatingwithin the range 400 to 500 F.

It is to be understood that the invention is not limited to the specificoperations and compositions herein described but may be carried out inother ways without departure from its spirit.

What we claim is:

1. A coated steel article, comprising a steel substrate, an intermediateadherent coating of gold or silver or copper or brass, and an outeradherent coating of zinc, in which there is an iron content in thecoatings, occasioned by alloying between the iron in the steel and thecoatings thereon, said iron content being less than about 100 milligramsper square foot of surface.

